The filter plug in a filter cigarette is commonly made from cellulose acetate tow or like material. Because such material tends to have a high bulk surface it is difficult to control the dimensions of a plug made solely of such material; thus, it is conventional to enclose the longitudinal, generally cylindrical, surface of the filter plug with a fibrous bonded sheet material, commonly referred to as "plug wrap". In the manufacture of filter cigarettes, typically a filter rod is produced from continuous filter tow and plug wrap. During this process the continuous rod is cut into a configuration of 6-up or 4-up filter lengths. These are transferred to a cigarette machine where they are further cut into double filter lengths, each being equivalent to two cigarette filters. During cigarette assembly a tobacco rod is applied to each end of the double filter length, tipping paper is then applied to the cut length of filter rod, which is then cut to form the discrete cigarettes.
Owing to legislative and other pressures to reduce the level of tar in cigarette smoke, it is now optional to ventilate filter cigarettes by providing the tipping paper with tiny apertures (also referred to as "pores" or "micropores"), the size and number of which may be varied according to requirements. The air admitted into the filter plug through the apertures in the tipping paper exerts a dilution effect, thereby reducing the concentration of tar in the smoke. Furthermore, this additional air allows the smoker to draw more easily on the cigarette.
In describing this invention the term "air permeability" will be used to describe the ability for air to pass through a material when a pressure differential is applied across the material. It is, however, common parlance to use the term "porosity" where strictly speaking "air permeability" is more correct. Where specific reference to types or grades of paper is made herein then commonly accepted terminology is used, e.g. "high porosity" porous plug wrap paper.
It is conventional to employ a non-porous or low- to medium-porosity paper as the plug wrap in full flavoured medium- to high-tar filtered cigarettes; however, the move to ventilated low tar cigarettes gives rise to a need for a plug wrap material that exhibits a higher permeability to air, which is generally achieved by incorporating into the plug wrap material fibres that are of different dimensions than the usual papermaking fibres. However, the use of higher permeability plug wrap material gives rise to problems in the conversion of the sheet material used for the plug wrap into the finished filter rod.
In the production of the filter rod, a continuous tow of fibres is pulled from a bale, the tow is then spread open, a plasticiser is applied and the tow is then brought into the form of a rod of the required diameter; the tow density (derived from the number of filaments in the tow and the respective tex of each filament) is also controlled, since this largely determines the filtration characteristics of the filter plug. The plug wrap is taken from a narrow bobbin or reel (typically one inch (2.54 cm) in width and containing typically 5000 linear meters), an adhesive is applied as a thin bead along a central line to anchor the cylindrical filter tow, with another, and usually more substantial, bead of adhesive being applied at the edge. The beads of adhesive are typically applied through nozzles, although other application means may be used. The plug wrap material is then folded into a tube enclosing the rod of tow material. The adhesive applied to the plug wrap sheet material in the above process is commonly a hot-melt adhesive, although it may be desirable to use a water-based adhesive, alone or in combination with a hot-melt adhesive, with a view to ensuring a good bond, reducing costs, and reducing the likelihood of plasticiser/adhesive interaction (which may be a concern when there is likely to be a long transit time between manufacture and point of sale).
The resultant tube, filled with the tow material, is passed through a garniture, which is a device having a conduit which brings the tube to the required diameter for the finished filter rod. The garniture is also commonly cooled in order to achieve satisfactory bonding of the hot-melt adhesive despite the rapid throughput. When water-based adhesives are employed, it may be appropriate, instead, to heat the garniture in order to effect bonding.
A problem that arises when using a porous or permeable sheet material for the plug wrap is the increased tendency for the adhesive to pass through the sheet ("bleedthrough"), which may cause a build-up of adhesive in the garniture and, indeed, on other parts of the forming apparatus. A build-up of adhesive in such parts as the garniture rod former and pass tubes may cause such deformities in the rod as creasing or dimpling, which may adversely affect the overall cigarette quality. Furthermore, the build-up of adhesive within the garniture may result in variation in filter-rod diameter. Both the deformities such as indentations and the differentials in rod diameter during cigarette assembly will affect the overall ventilation/dilution characteristics of the finished cigarette. The build-up of adhesive can even be so severe as to cause a blockage or restriction of the sealed rod passage, resulting in machine stoppage. In any case, the problem of adhesive bleedthrough and build-up during filter making can lead to poorer process efficiency in view of the need to interrupt the manufacturing process in order to remove the deposits of adhesive.
Accordingly, there is a need in the art for a fibrous bonded sheet material of high air permeability which nonetheless can meet the demands of high-speed converting units and which is not susceptible to excessive bleedthrough of adhesive (preferably whilst allowing sufficient penetration of the adhesive into the sheet material to ensure a good bond).
It has proved difficult to tackle the problem of the bleedthrough of adhesive by using cross-linking agents in the adhesive in order to speed up the curing rate; the cross-linking agents that could be used would generally not be acceptable in a product that comes into contact with the mouth.